What are the top 5 can't miss tourist attractions in S.F.?

Why settle for one.  If I were you, I would go to the The Buena Vista Cafe for an irish coffee.  From there (or close to there) you can see the Golden Gate Bridge, walk past the end of the line for the Powell-Hyde Cable Car (seeing it is good enough for me, but you can ride it if you choose), then walk down to Fisherman's Wharf.  An easy way to hit all three.  An alternative (or addition) to Fisherman's Wharf would be the Ferry Building Marketplace which is a super high end mall with a bunch of cool farmers' market type shops and stalls, restaurants, etc., and is right on the water.

If you had to narrow it down to 5 things (which is so hard to do in this tie-dyed, technicolor wonderland), I'd first explore the following 3 neighborhoods:

1. Mission District: This is one of the coolest neighborhoods in San Francisco. Murals and street art adorn the buildings, colorful Latino markets line the streets, and upscale restaurants sit side-by-side with dive bars and street food vendors. Working class folks, dot-com millionaires, hipsters, activists, and artists all make this place their home. Some specific highlights: Dolores Park, Balmy Alley, Misíon San Francisco de Asís, eat a burrito at Pancho Villa Taqueria, get coffee at Stanza Coffee Bar.
2. Haight-Ashbury: This iconic neighborhood was home to the hippies that pioneered the 1960’s counterculture movement and introduced America to open sexuality, mind-altering drugs, and psychedelic rock and roll. This neighborhood was home to rock legends such as Janis Joplin and the Grateful Dead. If you're here, wander in to Golden Gate Park as well.
3. North Beach (California): This Italian-centric neighborhood has a European vibe and is the perfect spot to sip a cappuccino on the street, explore boutique shops, listen to live music, and hit the town, as it features a variety of dining and nightlife options. Some specific highlights: check out the views from Coit Tower, explore City Lights Bookstore, sip a cappucino at one of the street side cafes on Columbus, get a pizza at Tomasso's, explore a few boutiques, and wander into Chinatown if you have time.

Then, these are 2 tourist sites you can't miss:

1. Golden Gate Bridge (either go to the bridge or check it out the views from Baker Beach): This 1.7 mile long bridge spans the Golden Gate strait, and was once the largest suspension bridge in the world. Initially considered the “bridge that couldn’t be built,” the Golden Gate Bridge is now one of the seven Wonders of the Modern World and the most internationally recognized symbol for California. 
   
2. Ferry Building: This historic ferry terminal serves as an indoor market filled with renowned restaurants and specialty food purveyors, selling artisan cheese, locally roasted coffee, California wine, craft beer, gourmet ice cream, and more. Daily ferries to Oakland, Sausalito, Tiburon, and other destinations across the bay frequently arrive and depart here as well.

Three honorable mentions are the Golden Gate Park, the Painted Ladies in Alamo Square and Alcatraz. More importantly, the food and wine are incredible throughout the city.

If you want a self-guided walking tour of the city on your iPhone. We have an awesome local guide that will tell you stories about all the must-see sites and neighborhoods at your own schedule and pace. Check out TripScout.co to learn more or download the app here.

You must go to Muir Woods National MonumentThat place is pure heaven! You will remember it for the rest of your life and want to go back!

Fisherman's Wharf tends to be fairly touristy, but I still recommend going there! you can take the Powell street cable car all the way there from the Powell BART(train) station. Definitely ride the cable car up this street! It gives you a good quick-view of the city.  In the wharf, you can see the seals, Pier 39, Alcatraz, and the Golden Gate Bridge! This spot just has a great view of the water and great choices for seafood. There is just so much to do here. If you're a chocolate lover like I am, you should go to Ghiradelli Square and have a sundae! Hope this helps.

Buy a ticket for a Hop on and Hop off bus. Get the bus on the Embarcadero Center and in a few hours you can see the whole city: Golden Gate Bridge, Cliff House, Ocean Beach, Golden Gate Park, Twin Peaks, the Castro, Civic Center, Union Square, San Francisco Chinatown, North Beach (California). Once you have seen the highlights you can decide what you would like to explore in the time remaining. Why settle for one "tourist trap?" For a few $ investment you can easily grasp the whole city. My personal favorite "tourist trap" is drinks at The Top of the Mark on Nob Hill. It is a classic view that never gets old.

Get a sandwich at Marina Submarine on Union St. near Steiner. Q, the chef is a master. Be sure to get expertly sliced avocado.

Only one person mentioned the Palace of Fine Arts in the area.  For me it is a fabulous no cost stop I wouldn't miss. 

Golden Gate Bridge hands down.  Avoid Fisherman's Wharf. Total waste of time.

You can do all of those things in 24 hours. San Francisco is a small big city.

With limited time, I would explore the Northern edge of San Francisco. Starting from the Presidio, I would do the following:

1. Rent bikes from Sports Basement
2. Bike by Crissy Field and check out the views of the Golden Gate Bridge
3. Coffee break at Warming Hut Cafe and Torpedo Wharf
   
4. Bike up to Golden Gate Bridge Pavilion and across the bridge. 
5. Bike into Sausalito and enjoy some tacos at Salsalito Taco Shop .
   
6. Take the ferry from Sausalito Ferry Landing back to Pier 39 . 
7. Have some clam chowder at Chowders and look at the Sea Lion Colony . 
8. Bike through Fisherman's Wharf and stop by Musée Mécanique for some old school games. 
9. Bike by Aquatic Park and Ghiradelli Square and Fort Mason .
10. Bike through Palace of Fine Arts at night for a gorgeous view. 
11. Return the bike at Sports Basement . 
12. Have dinner at one of the great restaurants in San Francisco. 

This is a version of the tour I did, with a couple extra stops.

https://sftravelplanner.wordpress.com/2014/11/10/andy-and-suns-day-of-fun-golden-gate-bridge-sausalito-and-fishermans-wharf/

I also run at Meetup group and we host fun adventures throughout San Francisco. Check us out! 

http://www.meetup.com/San-Francisco-Urban-Adventure-Club

I would say riding the cable cars

Alcatraz.  You get a nice view of San Francisco Bay on the boat ride, the prison's history is fascinating and the tours are a lot of fun.

Definitely a hop on hop off bus tour! You get a very good over view of the city and a ride across the Golden Gate Bridge. We also did a night tour which showed you different sights. You then have a choice of going back and doing or seeing something from the tour in more depth. There is so much to see and experience in this city and this is a good way of gaining a cross section of it!

Golden Gate Park is truly a national treasure. The De Young Museum is world class.

Fisherman's Wharf is pretty family-oriented.  The bridge is nice, if you like cycling at all and the weather is decent, it's nice to rent a bike at the Wharf and bike across the bridge. It's not for the faint of heart, though, particularly if you bike both ways. Taking the ferry back from Sausalito is nicer, in my opinion.  That said, this could easily take up your entire day.  A better bet might be a short cruise around the Bay, they leave from the Wharf and give nice views of the bridge.  Then you could get to do a bit of both.

If you have 24 hours in SF you have time for more than one of these things. I recommend that you start the day walking half way across the Golden Gate Bridge (about an hour), then we like to walk around Union Square and eat lunch, catch a cable car down to Fisherman's Wharf to explore and eat dinner. Then head back to Union Square by cab or cable car.

My vote would be a tour of Alcatraz, because you can see a lot of that when doing the tour. You go to Fisherman's Wharfto take the boat and then you get views of Golden Gate Bridge from the boat ride and on the island. I lived in San Francisco for a couple years and that was my favorite touristy thing to do, especially the night tour of Alcatraz. It's a really unique experience and the tour is just incredibly interesting if you're into history. 

Yes, you can see a lot in 24 hours. It's a compact city. I love the Palace of Fine Arts. Check out a photo on line. It's beautiful. It's also close to Golden Gate Bridge. And you could also visit Fort Point National Historic Site at the base of the bridge, close to Fisherman's Wharf. I also love driving through the Presidio.

Choose Golden Gate Bridge